1. Field of the Invention
The present invention relates to an image forming apparatus including a fixing part for heat-fixing a toner image on a recording sheet.
2. Description of the Related Art
Conventionally, a heat roller type heat fixing apparatus using a halogen heater as the heat source thereof or a film heating type heat fixing apparatus using a ceramic heater as the heat source thereof has been used as a fixing apparatus for heating and fixing a toner image in an image forming apparatus, including a copier or a laser beam printer.
In general, a heater is connected to an AC power supply through a switching element, such as a gate-controlled semiconductor switch. Power is supplied to the heater using this AC power supply. A temperature detection element, for example, a thermistor as a temperature-sensitive element is provided in the fixing apparatus and, thereby, the temperature thereof is detected. The switching element is on/off-controlled based on the detected temperature to turn on/off a supply of power to the heater. Thus, the fixing apparatus is temperature-controlled so as to be set to a target temperature. The heater is on/off-controlled by means of phase-control or wavenumber-control.
Phase-control is a method of control in which power is supplied to the heater by turning on the heater at any phase angle within one half-wave of the AC power supply. On the other hand, wavenumber-control is a method of power control in which the heater is turned on/off in units of half-waves of the AC power supply. Conventionally, either the above-described phase-control or wavenumber-control has been used in most cases.
Reasons for selecting phase-control include suppressing blinks, i.e., flickers in lighting equipment. A flicker refers to a phenomenon in which a voltage variation occurs in the AC power supply due to the load current fluctuation of electric equipment connected to the same power supply as that of the lighting equipment and the impedance of distribution lines, thus causing the lighting equipment to flicker. Since a current is turned on in units of half-waves in phase-control, the amount of current change is small and the cycle thereof is short. Consequently, flickers can be prevented from being emitted. On the other hand, in wavenumber-control, on/off-control is performed in units of half-waves of the AC power supply. Accordingly, a current variation becomes larger in wavenumber-control than in phase-control and, therefore, flickers are liable to occur.
Reasons for selecting wavenumber-control include suppressing harmonic currents and switching noise. Harmonic currents and switching noise occur due to a sudden current variation caused when the heater is turned on/off. Wavenumber-control is selected because these harmonic currents and switching noise are less likely to occur in this control, in which the heater is always on/off-controlled at zero-crossing points, than in phase-control in which the heater is switched on/off partway through a half-wave of the AC power supply. These harmonic currents and switching noise tend to be greater in magnitude in proportion to the voltage of an AC power supply used.
Accordingly, the control method of the heater is generally fixed according to an AC commercial power supply voltage in a district where an image forming apparatus is used. For example, a phase-control system advantageous against flickers is adopted for a district using an AC commercial power supply voltage of 100 to 120 V. In contrast, a wavenumber-control system advantageous against harmonic currents and switching noise is adopted for a district using an AC commercial power supply voltage of 220 to 240 V.
However, some literature proposes a system that combines phase-control and wavenumber-control. For example, according to Japanese Patent Application Laid-Open No. 2003-123941, phase-control is performed for some half-waves, among a plurality of half-waves defined as one control cycle, and wavenumber-control is performed for the rest of the half-waves. This system enables the suppression of harmonic currents and switching noise to a greater extent, compared with a system using phase-control alone. In addition, Japanese Patent Application Laid-Open No. 2003-123941 states that flickers can be reduced to a greater extent and power to the heater can be controlled in a greater number of steps, compared with a system using wavenumber-control alone.
Power supplied to a heater is increasing steadily due to a recent increase in printing speed. In addition, it has become increasingly difficult to comply with regulations on flicker and harmonic current emissions by means of conventional heater power control using phase-control or wavenumber-control alone, due to the tightening of such regulations. On the other hand, such a control system that combines phase-control and wavenumber-control as described in Japanese Patent Application Laid-Open No. 2003-123941 is effective.
It is difficult, however, to comply with various regulations and reduce adverse effects on external equipment in every situation where power supplied to a heater increases further and variations are present in a commercial AC power supply voltage, a heater resistance value, and the like, simply by combining phase-control and wavenumber-control.